Sorry, no idea. I haven't used MongoMapper for about 2 years now. Also haven't followed devise internals since I created this gem. Feel free to patch it as you see fix or do whatever hacks u find necessary :)

Not sure if this is right place to fix this. Such bug occurs because MongoMapper::Document instances don't respond to #assign_attributes used by Devise. In my opinion fixing this should be done in orm_adapter gem or even in mongo_mapper (AR instances responds to this method)

Allows you to set all the attributes for a particular mass-assignment security role by passing in a hash of attributes with keys matching the attribute names (which again matches the column names) and the role name using the :as option.

To bypass mass-assignment security you can use the :without_protection => true option